Beneficial Use of Dredged Material in Brown County, Wisconsin
A problem is becoming an opportunity when one considers what’s happening to dredged material in Wisconsin’s Brown County. The county is utilizing a straight forward process to dewater dredged material from the Port of Green Bay and make it available as a resource for agricultural and transportation purposes. This action may eliminate the need to build future disposal capacity.
Dredging and Disposal Situation
The United States Army Corps of Engineers (USACE) dredges approximately 150,000 cubic yards of sediment annually to maintain an 18 mile long shipping channel in the Port of Green Bay at the designed depths of 22-26 feet. In the early 70’s, a majority of the sediment was dredged hydraulically and pumped to the Bay Port disposal facility. After it was filled to its initially proposed elevation, Brown County received authorization to additionally fill within a 160-acre area of the 400 acres with sediment hauled by truck. Also an in-water confined disposal facility (Renard Island also known as Kidney Island CDF) received 1.2 million cubic yards of sediment under the Public Law 91-611 enabling legislation.
As the island CDF was nearing capacity, the Brown County Harbor Commission accelerated its search for disposal alternatives. The desirable option of continued use of the Bay Port facility would require periodic removal of material, preferably for beneficial use. Such an action would require a part of the facility to be designated as a landfill. The Wisconsin Department of Natural Resources (DNR) and the County worked together to expedite the landfill approval process and construction began in 1997.
Bay Port Landfill Design
Before the harbor commission could implement beneficial use options, it was necessary to find an economic method to dewater the sediments. The result was a design that used four cells for dewatering and two for storage or disposal of the dewatered sediments. With funding support from Wisconsin’s Department of Transportation ($1.3 million) and U.S. EPA’s Great Lakes National Program Office ($150,000), a facility was built which occupies 110 acres and is designed to accept over 200,000 cubic yards of wet sediment yearly, with a total dry sediment capacity of 2 million cubic yards. The dewatering cells range from 7 to 20 acres in size, with perimeter berms that are up to 15 feet high. The perimeter berms are constructed with a clay core with the remainder of the berm constructed of on-site soils. There are 1 to 3 off-loading ramps per cell depending upon the size of the cell.
The base of each cell is graded to one corner where a gravel dewatering infiltration bed and discharge piping drain off carriage water. The off-loading ramps are constructed on the high corners of each cell so that the sediment can flow across the cell, creating a slope on the surface of the sediment that allows precipitation to drain to the low corner. The infiltration bed is covered with coarse sand to prevent the sediments from draining through the gravel, yet permit the gradual drainage of water. This water leaves the cell through a pipe that drains into ditches that connect to a series of sedimentation ponds, and eventually to the waters of Green Bay. The DNR approved a stormwater pollution prevention plan that includes monthly monitoring for total suspended solids and phosphorus and quarterly monitoring for PCB’s.
In general, the facility will operate on a three year cycle, with 150,000 cubic yards deposited in the first cell in year one and the dewatered sediments removed after two years. The dewatered sediments will be excavated with conventional earth moving equipment and transported to one of two storage/disposal cells where it will be stockpiled, graded, and vegetated. The two storage/disposal cells can hold 600,000 cubic yards of dewatered and compacted sediment, which is equivalent of 12 years of dredging. The facility can operate for an additional 38 years by over-filling the storage cells and then filling the dewatering cells with this soil.
Beneficial Use Opportunities
Brown County is confident that the facility will never fill because they have embarked on an aggressive program of developing beneficial use options. In 1995, two-year-old sediment from Bay Port was used in a greenhouse study. This project included a bulk analysis of the sediment, leaching tests, and plant growth and chemical uptake analysis. The data was compared to the various standards for groundwater, hazardous waste, wastewater solids, and fertilizer value. Except for an elevated concentration of nitrogen, the soil passed all of the other standards. Because of the increased nitrogen, the study measured a 50% increase in plant weight over plants grown on a control plot. This attribute will be used as a selling point for the use of this material as a soil amendment and fertilizer.
Another beneficial use opportunity studied was to use the soil as general fill, in cooperation with the county and state transportation departments. This study, funded through Wisconsin’s Coastal Management Program, evaluated the practicality of dewatering the sediment and using it in a road embankment. The study showed that it was not necessary to actively dewater sediments that had been deposited four years previously. Instead, the contractor was able to scrape 6-12 inches of soil off the fill periodically to generate a 10,000 cubic yard stockpile of suitable soil. Some additional work remains, however, to determine whether the sediment typically dredged from Green Bay harbor is structurally suitable for road construction and at a cost that is competitive with other alternatives.
A project the county is currently working on with the USACE is composting sediment with cow manure and chipped shipping pallets. The work is a continuation of a composting project that the USACE conducted at the Milwaukee harbor in 1998 with funding support from EPA’s Great Lakes National Program Office. The Corps project will include an extensive greenhouse study and chemical testing. The goal of the composting project will be to determine how composting degrades PCB’s that are contained in the sediments.
The Brown County Harbor Commission has aggressively pursued unique solutions to sediment disposal. The commission’s attitude has always been that the sediment is a resource that should be used, not buried. A practical goal of the commission is to reuse every cubic yard of sediment dredged from the river. Everyone acknowledges that the ultimate goal though would be to be to drastically reduce the need to dredge by eliminating upstream soil erosion.
Note: This case study was prepared during the summer of 1999.